Transition detector circuit

ABSTRACT

A transition detector circuit comprises a first invertor train comprising 2n stages of invertors (n: positive integer including zero), the input thereof being connected to a signal input terminal while the output thereof is connected to an in-phase output terminal, a second invertor train comprising 2n+1 stages of invertors, the input thereof being connected to the signal input terminal, while the output thereof is connected to an antiphase output terminal, a third invertor train comprising at least one stage of an invertor, which is connected to the output of the first invertor train, a fourth invertor train, comprising at least one stage of an invertor, which is connected to the output of the second invertor train, and a fifth invertor train comprising at least one stage of an invertor, which is connected to the signal input terminal. The transition detector circuit further comprises a first switching circuit which turns on or off in response to a signal fed from the third invertor train to produce a pulse signal having a first delay time determined by the first and third invertor trains, and a second switching circuit which turns on or off in response to a signal fed from the fourth invertor train and turns off or on in response to a signal fed from the fifth invertor train to produce a pulse signal having a second delay time determined by the second, fourth and fifth invertor trains.

BACKGROUND OF THE INVENTION

The present invention relates to a MOS (MetalOxide-Silicon) integrated circuit provided on a semiconductor substrate, and more particularly to a transition detector circuit for sensing a change of an input signal of the MOS circuit, thereby producing pulse signals responsive thereto.

Heretofore, there have been proposed some conventional transition detector circuits. One is a circuit including a feedback network for effecting input control. The other is a circuit which does not include such a feedback network.

The problems with the transition detector circuit including a feedback network are that the circuit design thereof is complicated and there occurs undesirable time delays in the operation thereof.

On the other hand, the transition detector circuit without a feedback network is preferable in that the circuit design is simple and operates at a high speed. However, it happens that this circuit operates erroneously due to the difference of operation timing of each circuit component, when input signals vary slowly.

SUMMARY OF THE INVENTION

With the above in mind, an object of the present invention is to provide a transition detector circuit making it possible to operate at a high speed with a simplified circuit construction and to effect a normal operation, even if there exists a limited timing relationship or timing limitation between an input signal and an output signal.

In order to achieve this object, according to the present invention, there is provided a transition detector circuit capable of producing output pulse signals each having a desired pulse width by means of a circuit which is simplified and operates at a high speed, in which a feedback network is eliminated and a plurality of invertor trains are provided, and capable of effecting a normal operation even if there exists a timing limitation between an input signal and an output signal on the basis of the circuit construction that an invertor or some invertors among invertors for limiting a pulse width of the output pulse is in common with respective invertor trains for producing in-phase and antiphase outputs and that a predetermined relationship holds between threshold levels of these invertors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are circuit diagrams illustrating prior art transition detectors, respectively;

FIG. 3 is a timing chart for explaining a timing limitation between an input signal and an output signal of FIG. 2 circuit;

FIG. 4 is a circuit diagram illustrating an embodiment of the present invention;

FIGS. 5 and 6 are timing charts showing operations of the FIG. 4 circuit, respectively; and

FIG. 7 is a circuit diagram illustrating another embodiment of the invention.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Prior to the description of the invention, reference will be first made to the conventional transition detectors in conjunction with FIGS. 1 to 3. Thereafter, the preferred embodiments of a transition detector circuit according to the present invention will be referred to in conjunction with FIGS. 4 to 7.

The prior art transition detector in a MOS integrated circuit is now described with reference to FIGS. 1 to 3.

FIG. 1 shows an example of circuit of a conventional device, as shown in U.S. Pat. No. 4,039,858. An output signal 1 of the transition detector is utilized in order to effect input control for an input signal 2. The transition detector is designed so as to produce pulse signals at an output terminal 3 with the feedback looped network thus formed. However, since the circuit construction shown in FIG. 1 requires the feedback network, its circuit design becomes complicated. Further, with this circuit, output signals 4 and 5 except for pulse signals are output through a transfer gate controlled by the pulse signal 1, thereby resulting in large time delays.

FIG. 2 is a circuit diagram illustrating another conventional device. A feature of this embodiment is that the feedback network as employed in FIG. 1 circuit is not used, and time interval adjustment is effected depending on the stage number of an invertor train. Between a signal input terminal 6 and an in-phase output terminal 7, there is provided an invertor train comprising an even number of invertor I₁ and I₂. On the other hand, between the input terminal 6 and an antiphase (180° out-of-phase) output terminal 8, there is provided an invertor train comprising an odd or uneven number of invertors I₃, I₄ and I₅. A first pair of Field Effect Transistors (which will be called "transistors" hereinafter) T₀₁ and T₀₂ connected in series, and a second pair of transistors T₀₃ and T₀₄ connected in series are connected to one reference power supply V_(SS), respectively. These pairs of transistors are further connected commonly to a pulse output terminal 9 through an invertor I₆. Another reference power supply V_(DD) is connected to the input of the invertor I.sub. 6 through a load 10. Further, an invertor train comprising I₇, I₈, I₉ and I₁₀ is connected to the signal input terminal 6. The outputs of invertors I₇, I₉ and I₁₀ are fed to the gate terminals of transistors T₀₃, T₀₂ and T₀₄, respectively. Further, a signal being propagated from the signal input terminal 6 is supplied to the gate terminal of the transistor T₀₁.

With the circuit construction shown in FIG. 2, a time interval of the pulse output C is determined due to signal delays occurring when the signal input A is propagated through three stages of invertors comprising either of invertors I₇, I₈ and I₉ and invertors I₈, I₉ and I₁₀ with respect to a change of the signal input A. As a result, this makes it possible to effect an adjustment of the time interval depending on the stage number of the invertor train, thereby making it easy to design the circuit. Further, signal propagating routes for the in-phase output B and the antiphase output B are different from that of the pulse output C. Consequently, this ensures that the circuit operates at a high speed.

However, as described above, in regard to the circuit construction shown in FIG. 2, the signal paths for the in-phase output B and the antiphase output B are different from that of the pulse output C. Accordingly, the circuit of FIG. 2 cannot properly operate when the following relationship is required in connection with the timing relationship between the in-phase and antiphase outputs B and B and the pulse output C. The reason therefore will be described with reference to FIG. 3.

As shown in FIG. 3, suppose the relationship in connection with time is as follows: It is to be noted that "H" or "L" showing a logical level is referred to in terms of usual logical expression. The designation t₁ denotes a time interval from a time when the signal input A changes from "L" to "H" to a time when the pulse output C rises, t₂ a time interval from the same time to a time when the antiphase output B changes to "L", t₃ a time interval from the same time to a time when the in-phase output B changes to "H", and t₄ a time interval from the same time to a time when the pulse output C falls. Likewise, t₅ denotes a time interval from a time when the signal input A changes from "H" to "L" to a time when the pulse output C rises, t₆ a time interval from the same time to a time when the in-phase output B changes to "L", t₇ a time interval from the same time to a time when the antiphase output B changes to "H", and t₈ a time interval from the same time to a time when the pulse output C falls. Further, suppose that the threshold voltages of invertors I₁, I₃ and I₇ and the transistor T₀₁ are V₁, V₃, V₇ and V.sub. 01, respectively. In this instance, when an inequality t₁ <t₂ <t₃ <t₄ and an inequality t₅ <t₆ <t₇ <t₈ must hold, an inequality V₀₁ <V₁ <V₇ is required from the former (t₁ <t₂ <t₃ <t₄), and an inequality V₃ <V₇ and an inequality (the time delay of invertors I₃ to I₅)<(the time delay of invertors I₇ to I₁₀) are required from the latter (t₅ <t₆ <t₇ <t₈).

Assume now that the signal input A varies slowly from "H" to "L". Since the output of the invertor I₇ changes and then the output of the invertor I3 changes, the relationship t₅ <t₆ is satisfied. However, in this instance, care must be taken. When the signal input A slowly changes and reaches the threshold level of the invertor I₇. After the change of the output of the invertor I₇ is propagated to the invertor I₁₀, the signal input A reaches the threshold level of the invertor I₃. Taking into account these conditions, the information in connection with the output change of the invertor I₇ is propagated to the invertor I₁₀. As a result, after the pulse output C has fallen, the output of the invertor I₃ changes. Consequently, the relationship t₇ <t₈ cannot be satisfied.

The preferred embodiments according to the invention is now described with reference to FIGS. 4 to 7. FIG. 4 shows a circuit diagram illustrating an embodiment of the invention wherein like components in FIG. 2 are designated by like reference numerals. The in-phase output being propagated from the invertor I₂ is fed to the gate of an n-channel transistor T₁₂ through the invertor I₁₁. The antiphase output being propagated from the invertor I₅ is fed to the gate of an n-channel transistor T₁₄ through the invertor I₁₂. The signal input A is fed to the gate of an n-channel transistor T₁₁, and is also fed to an n-channel transistor T₁₃ through the invertor I₇, respectively. The capacity of each invertor serving as a driver and the load 10 is determined so that the relationship V₇ >V₁ >V₃ V₁₁ holds between the threshold voltages V₁, V₃ and V₇ of invertors I₁, I₃ and I₇ and the threshold voltage V₁₁ of the transistor T₁₁. In order to obtain the relationship in regard to such threshold voltages, the dimension of the p-channel and n-channel of the transistors constituting invertors and that of transistors constituting a switching circuit are determined.

The operation when the signal input A changes to "L" for a second time after the signal input A changes from "L" to "H" in FIG. 4 circuit will be described with reference to FIG. 5.

When the signal input A is "L", outputs of invertors I₇ and I₁₁ or "H", and the output of inverter I₁₂ is "L". Accordingly, transistors T₁₁, T₁₂, T₁₃ and T₁₄ are placed in OFF, ON, ON and OFF states, respectively. As a result, the pulse output C is "L".

On the contrary, when the signal input A is changed to "H", the transistor T₁₁ turns on. As a result, the pulse output C changes from "L" to "H". Further, after the delay time T₁ determined by the invertors I₁, I₂ and I₁₁ is passed, the output of the invertor I₁₁ changes from "H" to "L". As a result, since the transistor T₁₂ turns off, the pulse output C is returned to "L" from "H". Thus, a pulse having a time interval (T₁) determined by invertors I₁, I₂ and I₁₁ is output from the terminal 9.

Further, when the signal input A changes from "H" to "L", the invertor I₇ changes from "L" to "H" in a predetermined time delay inherent in the invertor I₇, with the result that the transistor T₁₃ turns on. As a result, the output C changes from "L" to "H". Further, after the delay time determined by invertors I₃, I₄, I₅ and I₁₂ is passed, the output of the invertor I₁₂ changes from "H" to "L", with the result that the transistor T₁₄ turns off. As a result, the pulse output C changes from "H" to "L". Thus, a pulse having a time interval (T₂) determined by invertors I₇, I₃, I₄, I₅ and I₁₂ is output from the terminal 9.

As stated above, a pulse width of the pulse output C is determined by the delay time of each invertor. Accordingly, it is possible to set the desired value of the pulse widths T₁ and T₂ by increasing or decreasing the stage number of the invertor, or changing the delay time thereof.

The operation will be described with reference to FIG. 6 when the signal input A varies slowly from "L" to "H", and then varies slowly from "H" to "L" for a second time. In this instance, the above-described relationship V₇ >V₁ >V₃ >V₁₁ holds in connection with the respective threshold voltages V₇, V₁, V₃ and V₁₁ of invertors I₇, I₁ and I₃, and the transistor T₁₁.

When the signal input A approaches gradually from "L" to "H", and then reaches the level of V₁₁, the transistor T₁₁ turns on and the pulse output C changes from "L" to "H".

When the signal input A further approaches "H" and then reaches the level of V₃, the output of the invertor I₃ becomes "L". Accordingly, the antiphase output B becomes "L" in a predetermined time delay determined by PG,11 invertors I₄ and I₅. Further, since the output of the invertor I₁₂ becomes "H", the transistor T₁₄ turns on.

When the signal input A further approaches "H" and then reaches the level of V₁ the output of the invertor I₁ becomes "L". Accordingly, the in-phase output B becomes "H" in a predetermined time delay of the invertor I₂. Further, since the output of the invertor I₁₁ becomes "L", the transistor T₁₂ turns off. However, since transistors T₁₃ and T₁₄ have already turned on, the pulse output C is maintained "H".

When the signal input A further approaches "H" and then reaches the level of V₇, the output of the invertor I₇ becomes "L", with the result that the transistor T₁₃ turns off. As a result, the pulse output C becomes "L". Thus, a pulse having a time interval T₁ is obtained.

Then, after a predetermined time is passed, when the signal input A approaches from "H" gradually to "L" and then reaches the level of V₇, the output of the invertor I₇ is inverted into "H". Accordingly, the transistor T₁₃ turns on, with the result that the pulse output C becomes "H".

When the signal input A further approaches "L" and then reaches the level of V₁, the output of the invertor I₁ becomes "H" while the output of the in-phase output B becomes "L". Since the invertor I₁₁ becomes "H", the transistor T₁₂ turns on.

When the signal input A further approaches "L" to reach the level of V₃, the output of the invertor I₃ becomes "H". As a result, the antiphase output B becomes "H". Since the output of the invertor I₁₂ becomes "L", the transistor T₁₄ turns off. However, since transistors T₁₁ and T₁₂ are maintained on, the pulse output C is also maintained "H".

Finally, when the signal input A further approaches "L" to reach the level of V₁₁, the transistor T₁₁ turns off. As a result, the pulse output C becomes "L". Thus, a pulse having a time interval T₂ is obtained.

Referring to FIG. 7, there is shown another embodiment of the invention, wherein like components in FIG. 4 are designated by like reference numerals. A p-channel transistor T₂₁ and a p-channel transistor T₂₃ are connected in parallel. A p-channel transistor T₂₂ and a p-channel transistor T₂₄ are also connected in parallel. These parallelly-connected pairs are connected in series. The signal input A is fed to the gate of the transistor T₂₁. The outputs of invertors I₇ I₁₁ and I₁₂ are fed to gates of transistors T₂₂, T₂₃ and T₂₄, respectively.

The threshold levels V₁ V₃ and V₇ of invertors I₁, I₃ and I₇ and the threshold level V₂₁ of the transistor T₂₁ are determined such that the relationship V₂₁ <V₃ <V₁ <V₇ holds. This is accomplished by changing the channel dimension of the transistor, as stated above. Thus, the circuit shown in FIG. 7 normally operates when there exists a timing limitation as shown in FIG. 3 in connection with input and output signals.

As is apparent from the foregoing description, according to the present invention, the feedback network as employed in the prior art device is not used, and a pulse width is determined based on invertor trains. Accordingly, this makes it possible to embody a transition detector which is easy to design and which operates at a high speed. Further, the transition detector of the invention determines a pulse width of the pulse being output on the basis of the delay time of an invertor. Accordingly, this makes it possible to adjust a pulse width and to stabilize the pulse width as desired.

Further, according to the present invention, invertors for limiting a pulse width form a part of invertors for obtaining the in-phase and antiphase outputs. A predetermined relationship is given between the threshold levels of invertors and switching elements (for example, transistor) constituting the switching circuit. Thus, it is possible to obtain the transition detector circuit which operates desirably, even if there exists a timing limit as shown in FIG. 3 between input and output signals. 

What is claimed is:
 1. A transition detector circuit comprising:(a) a first invertor train comprising 2n stages of invertors (n: positive integer including zero), the input thereof being connected to a signal input terminal while the output thereof is connected to an in-phase output terminal; (b) a second invertor train comprising 2n+1 stages of invertors, the input thereof being connected to said signal input terminal, while the output thereof is connected to an antiphase output terminal; (c) a third invertor train, comprising at least one stage of an invertor, connected to the output of said first invertor train; (d) a fourth invertor train, comprising at least one stage of an invertor, connected to the output of said second invertor train; (e) a fifth invertor train, comprising at least one stage of an invertor, connected to said signal input terminal; (f) a first switching circuit which turns on or off in response to a signal fed from said signal input terminal and turns off or on in response to a signal fed from said third invertor train to produce a pulse signal having a first delay time determined by said first and third invertor trains; and (g) a second switching circuit which turns on or off in response to a signal fed from said fourth invertor train and turns off or on in response to a signal fed from said fifth invertor train to produce a pulse signal having a second delay time determined by said second, fourth and fifth invertor trains.
 2. A transition detector circuit as defined in claim 1, wherein the following relationship holds

    V.sub.3 >V.sub.1 >V.sub.2 >V.sub.4,

where V₁ denotes a threshold voltage of a first stage of an invertor of said first invertor train, V₂ a threshold voltage of a first stage of an invertor of said second invertor train, V₃ a threshold voltage of a first stage of said fifth invertor train, and V₄ a voltage at a time when said first switching circuit turns on or off in response to a signal fed from said signal input terminal.
 3. A transition detector circuit as defined in claim 2, wherein the relationship in connection with the threshold voltages is formed by suitably selecting the dimension of channels of transistor constituting each of said invertors and said switching circuits. 